IAL will help strengthen fins, I believe and is good for their health in general. Salt is fine as it will help any wounds repair but is supposed to be good for oxygen exchange in the gills too.

As for medication, I don't know I would recommend any this long after. Initially, yes, but any open wounds should have healed by now. It is probably down to strength in the fins. It must be considered that they may never straighten though, but as it happened in an accident, at least it wont pass on to any spawn ^-^

He is gorgeous though XD

 

There's no rushing fin healing. BettaMummy87's advice is quite good. I wouldn't bother with salt, since the injury is so old. IAL and frequent water changes should do the job. Also, high quality food, such as live or frozen, will give him more strength.

The fins will never be perfect again, but they should regrow.